The present invention relates to a novel method for the preparation of a diorganochlorosilane or, in particular, a dialkylchlorosilane by the selective reduction of a dialkyldichlorosilane whereby only one of the two chlorine atoms present in the dialkyldichlorosilane is reduced and replaced with a hydrogen atom.
Dialkylchlorosilanes represented by the general formula R.sub.2 SiClH, where R is an alkyl group, are important monomeric starting materials in the silicone industry, widely employed, for example, in the manufacture of silicone fluids having silicon-bonded hydrogen atoms Si--H as functional groups in the main chain or at the terminals of the siloxane chains. The dialkylchlorosilanes are also useful as an intermediate compound in the synthetic preparation of various kinds of organosilicon compounds. Therefore, it has eagerly been desired to establish a method for economical preparation of such dialkylchlorosilanes.
Despite a large commercial demand for dialkylchlorosilanes, no economical manufacturing methods have been established. For example, dimethylchlorosilane has been supplied only as a by-product obtained in the synthesis of dimethyldichlorosilane. That is to say, dimethylchlorosilane is contained in a low boiling fraction from the synthesis of dimethyldichlorosilane and isolated by distillating the fraction.
A possible synthetic procedure for a dialkylchlorosilane may be the selective reduction of one of the two chlorine atoms in the corresponding dialkyldichlorosilane to convert the Si--Cl linkages to Si--H, using a suitable reducing agent, such as lithium aluminum hydride, which is known to be effective for reducing the Si--Cl to Si--H. Lithium aluminum hydride has, however, no satisfactory selectivity in the reduction of dialkyldichlorosilane producing dialkylsilane, R.sub.2 SiH.sub.2, by the dechlorination reduction of both of the two chlorine atoms in the starting silane. Thus no reducing agent having a sufficient selectivity has been available for the direct reduction of dialkyldichlorosilanes to the desired dialkylchlorosilanes.
Several alternative methods have been reported in literature for the preparation of dimethylchlorosilane. For example, a polydimethylsilane having --(CH.sub.3).sub.2 Si-- units as the recurring units is subjected to a cleavage reaction by passing hydrogen chloride gas under irradiation with ultraviolet light (see Chemical Communications, 1970, p. 507) to produce dimethylchlorosilane. As another example, dimethyldichlorosilane is reacted with diethylamine to produce dimethyl(N,N-diethylamino)chlorosilane which is reduced with lithium aluminum hydride to dimethylchlorosilane (see Journal of Organometallic Chemistry, 18 (1969), P. 371). As a further example, dimethyldichlorosilane is reduced with sodium hydride in the presence of aluminum chloride to dimethylchlorosilane (see Zhurnal Obshchei Khimii, 40 (1970), p. 812). These synthetic methods, however, are disadvantaged by the complexity of the reaction procedures that require special skills to achieve satisfactory results.